Organic bonded abrasive article



1966 J. R. THOMPSON ORGANIC BONDED ABRASIVE ARTICLE Filed Aug. 14, 1964 FIG. 2

JOHN R. THOMPSON IN VENTOR.

United States Patent 3,283,448 ORGANIC BONDED ABRASIVE ARTICLE John R. Thompson, Westboro, Mass., assignor to Bay State Abrasive Products Company, Westboro, Mass., a corporation of Massachusetts Filed Aug. 14, 1964, Ser. No. 389,575 7 Claims. (Cl. 51--206) This invention relates to an organic bonded abrasive article and, more particularly, to an abrasive formulation which may be adapted to the use of diamond abrasives as well as less expensive conventional abrasives, and an organic bonded wheel formulation which may be used for both conventional and electrolytic grinding.

In the removal of material by grinding, several methods have been developed over the years. In the conventional method, there is direct frictional contact between the moving wheel and the surface of the workpiece. The abrasive particles in the grinding wheel act as small individual cutting tools which remove chips of material from the workpiece. Care is taken to choose a grinding wheel which has a suitable bond for the particular grinding job to be done so that the maximum usable life may be obtained from the abrasive grains. In the case of premium price abrasives, such as diamonds, this is particularly important. Therefore, the bond must hold the abrasive grain firmly until the cutting points have been worn smooth and then the grain must be released, exposing new fresh cutting points. If the bond is not strong enough, the abrasive will be prematurely released, the wheel will give short life and accompanying high cost. If, on the other hand, the bond is too strong, the grains are not released from the grinding face, and the wheel glazes, loads, and stops cutting. In the electrolytic grinding process, on the other hand, the situation is quite different. Inelectrolytic grinding, both the matrix of the abrasive wheel and the workpiece must be capable of conducting electric current, while the abrasive particles which protrude from the face of the wheel act as insulating spacers to avoid the dangers of short circuits. Also, these protruding abrasive particles contribute a scrubbing or scraping action to remove the oxide film which is produced by the electrolytic action. The primary requirement of a bond for electrolytic grinding is that it be electrically conductive.

Although conventional grinding is done with grinding wheels bonded with organic materials (such as resin and/ or rubber), with sintered metals, or with vitrified bonding materials (such as clays, oxides, and mineral formulations), previously, the only wheels which have achieved any degree of success for electrolytic grinding have been those in the group bonded with sintered metal powders. There have been attempts to make conductive wheels by the treatment of vitrified Wheels after manufacture with a conductive substance or by using graphite fillers in organic bonded wheels, but these attempts have been largely unsuccessful. On the other hand, although there are many organic bonded diamond wheels on the market, one of the difficulties with this type of wheel is that, with the heat generated in grinding excessively hard materials, the organic bond thermally degrades and releases the diamond particle prematurely. These 'difliculties experienced with the prior art devices have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the invention to provide an abrasive wheel which may be used both A further object of the present invention'is the provision of an abrasive wheel which may be used in electrolytic grinding at very high current densities.

It is another object of the instant invention to provide a grinding wheel capable of very high thermal and electrical conductivity whereby its use in conventional or electrolytic grinding is enhanced.

It is a further object of the invention to provide a grinding wheel of the resinoid type having high thermal and electrical conductivity characteristics.

A still further object of this invention is the provision of a grinding wheel which is useful in electrolytic form grinding and in which the face may be shaped with comparative ease.

It is a still further object of the present invention to provide a grinding wheel useful in grinding a very hard material, such as tungsten carbide, in which the abrasive surface of the wheel is not unduly heated and in which wheel life is greatly extended.

Another object of the invention is the provision of a grinding wheel which, when used in conventional grinding, is capable of extremely long wheel life and, when used with electrolytic grinding, is capable of higher rates of stock removal.

It is a further object of this invention to provide a grinding wheel that produces a finer than usual finish on the workpiece because the grinding may be achieved with minimum vibration.

It is a further object of this invention to provide an abrasive wheel with a very similar composition in the rim section and the core section, with very similar or matched physical characteristics, thereby eliminating or greatly minimizing many of the problems of heat transfer and thermal expansion and separation which have been bothersome for many years. 7

With the foregoing and other objects in view, as will appear as the description proceeds, the invention resides in the combination and arrangement of parts and in the details of construction hereafter described and claimed, it being understood that changes in the precise embodiment of the invention herein disclosed may be made within the scope of what is claimed without departing from the spirit of the invention.

The character of the invention, however, may be best understood by reference to one of its structural forms, as illustrated by the accompanying drawings. For purposes of illustration, the wheel that is shown is known in the industry as a straight wheel, DlAl shape, but it should be understood that the invention wheel may be manufactured in any of the commonly known and used shapes, of which there are about 2,000 modifications.

FIG. 1 is a perspective view of an abrasive article embodying the principles of the present invention, and

FIG. 2 is a sectional view of the invention taken on the line IIII of FIG. 1:.

Referring to the drawings, it can be seen that the abra- .'SiV8 article, indicated generally by the reference numeral 10, consists of a wheel having a central core 11 and an outer rim 12. The core is provided with a central aperture 13 by which the wheel is mounted on the shaft of a grinding machine or the like. The rim 12 is provided with an outer cylindrical surface 14 which is adaptul to be placed in a predetermined geometric relationship to a workpiece surface to be finished while the wheel is rotated.

The materials used in carrying out this invention comprise metal powders selected from those which have a thermal conductivity expressed in cal./cm. /cm./sec./ C. of over .500 and an electrical resistivity expressed in microohms/cm. (at 86 F.) of less than 3.0. These powders may be pure metallic elements, or mixtures, or powders of alloy metals.

Patented Nov. 8, 1966 l l l The abrasives used may be diamonds, or any of the usual abrasive, for example, alumina in any form, including fused, natural, or sinteredaluminas, emery, other abrasive oxides, and certain hard carbides.

'The organic bonding material most commonly used is phenol-formaldehyde resin, for example, Bakeliteresin iproduced and supplied by Union Carbide Corporation, particularly those resins wnown as BRP 5417 or BRP5727. tHowever, one could substitute any of various other welllknown grinding whel resins of the melamine, urea-formjaldehyde, or other types. Mixtures of resins may also }be used according to known standard methods.

The use of metallic fillers in organic bonded abrasive Wheels is not, of itself, a new concept. However, among the new and surprising features of this invention is the fact that such a large proportion of metal filler can be successfully bonded by such an usually small proportion of resin and the resulting abrasive wheel still has an adequate safety factor for heavy duty grinding.

It is further surprising that, although the electricallyconducting metallic powders are bonded together with a ,resin which is, in itself, a non-conductor, the resulting wheel is extremely conductive and can be used at higher current density levels, and with much greater metal re- Ingredient: Wt. Percent Copper powder 80-95 Resin 520 Furfural is added as a wetting agent in the amount of 25 cc. per pound of powdered resin. One of the preferred resins for the core is Bakelite resin, BRP5417, although,

' undoubtedly, there are many other suitable well-known 1 reslns. In a preferred embodiment,-the copper powder is thoroughly blended with the proper amount of furfural, based 1 on the weight of resin according to thepreceding formula. 2 Then, the pre-weighed amount of powdered resin is added and mixed in thoroughly. This mixture is then pre-cured :for two hours at 225 F. in a shallow pan. It is then f granulated, molded on the desired shape, hot-pressed and i cured in a 21-hour curing cycle at a temperature not over 400 F. After curing, the preform is machined to the exact desired dimensions and, before assembly with the abrasive containing rim, is coated with cement.-

The rim section is composed of the following formula- 1 tion:

Ingredient: Wt. Percent j Copper powder '70-85 Resin 8-16 Abrasive 1-20 One of the preferred resins used in this formulation is =Bakelite, BRP5727.

In manufacture of the rim, the metallic powder and the abrasive are mixed together, furfural is added as a wetting :agent, and then the powdered resin or resins. This mixture is molded onto the core 11, hot pressed and cured in a 21-hour curing cycle at not over 400 F.

, When the wheel is manufactured using A1 0 as the abrasive, it may be manufactured with an abrasive-conataining rim, bonded to a non-abrasive core, in exactly the ,same manner as in the diamond wheel, or the entire wheel jmay be manufactured from a. single abrasive-containing formulation and made according to standard manufacturing procedures.

I When diamond wheels are manufactured in the method described above and with the proportions set forth, they :are successful in grinding tungsten carbide, in tool and cutter grinding, saw-tip grinding, and off-hand burr grinding. Cut-off applications of the wheel include slotting of carbide millers and glass cutting. In the grinding of tungsten carbide and in other heavy duty grinding operations that are customarily performed with resin bonded diamond wheels, considerable heat is generated, especial-, ly in dry grinding. This generated heat causes two def-' nite problems. First, excessive heat has the effect of weakening the resin bond, and the diamond abrasive particles are prematurely released with accompanying short wheel life. Then, secondly, when the rim and the core of the wheel are made of materials with different thermal expansion properties, a problem of rim separation is very common. sumer and the wheel manufacturer. By use of the present invention, the use of the high thermally conductive filler or fillers transfers the heat generated at the grinding surface into the core of the wheel, resulting in less temperature build-up in the resinoid diamond rim and greatly increased wheel life. Experience has shown that the materials which are most useful are those with a thermal conductivity above 0.500 cal./cm. /cm./sec./ C. atv

A study of the representative test results in various opformance tests, because all conditions are carefully stand ardized and controlled.

In the acceptance test tabulated below, the customers limit of acceptable wheel wear per unit time was 0.0045 inch.

In. A standard wheel gave 0.0047 A commercial wheel gave 0.0082

A wheel made in accordance with the pre- 0.0002

ferred embodiment of the invention gave 0.0005 0.0006

Average 0.00054 The following tests were carried under actual working conditions:

Type of Operation Standard Wheel Invention Wheel Carbide Tool Grinding 1960 pieces 2,862 pieces. Off-hand Burr Grinding. 37.2 hrs. life..." 58.5 hrs. lil'e. Carbide Saw Grinding 20,000 tips 35,000 tips.

In glass grinding, while the invention wheel has not shown exceptional life or speed of cut, it has demonstrated excellent, smooth, burr-fee cuts, thus overcoming a common problem in this type of grinding.

Electrolytic grinding techniques have normally utilized metal bonded abrasive wheels, while the present invention relates to a resin bonded wheel which is made electrically conductive by using copper, silver, or such powders or mixtures of powders as filler in the resin bond.

This results in a wheel which is capable of higher stock removal rates than is now possible with metal bonded wheels because it is possible to use higher voltage and amperage in the electrolytic process.

In one test of the electrolytic properties of the present invention, a resin-bonded wheel was used of the standard straight cup type in which the rim contained grit diamonds, and in which the core and rim were provided with copper powder as a filler in the proportions described above. This wheel was used on an Everite Model H4 This is annoying and expensive for the consingle point tool grinder with a wheel speed of 3450 r.p.m. and with electrolytic flood coolant grinding tungsten carbide. The workpieces were triangular in shape, being /2 inch on each side and 2 inches long. The first test was run with the standard metal bonded Wheel normally used in this operation at a current density of 8 volts and 80 amperes. The rate of stock removal was 0.005 6 cu. in./minute. Then the invention wheel was used under exactly the same conditions with about a increase in stock removal, namely 0.0059 cu. in./min. In this test, the standard metal bonded wheel was used at the maximum electrical energy which could be applied without excessive Sparking and a resulting loss in cutting rate. The invention wheel was then tested at a current level of 10 volts and 115 amperes without sparking, in which case the rate of stock removal increased by well over 80% to 0.0102 cu. in./min. There was no measuable wear on the test wheel.

A second electrolytic test was performed with an aluminum oxide wheel of the general type shown in the drawings. The diameter of the wheel was 8 inches, inch grinding face, with a 3 inch hole. The test involved form grinding of 400 series stainless steel pieces. The machine used was a Do-All 6 x 18 Surface Grinder equipped with an Everite power pack. Wheel speed was 5000 s.f.p.m. and electrolytic flood coolant was used. The workpieces were of an inch wide with a .687 inch radius. The workpiece thickness was .014 inch and the object of the operation was to achieve a smooth, completely burr-tree finish at a thickness of .009 inch.

Previously in this operation, standard metal bonded wheels have been used. These metal bonded wheels are manufactured with an approximate .687 radius on the grinding face. However, it is still necessary for the wheel to be precisely trued to the jig by dressing the face with a diamond dresser. In this instance, dressing took two 'hours to perform and, in the operation, the diamond dresser was ruined and the wheel edges were not as sharp as desired. The invention wheel, starting with a fiat inch grinding face, was trued properly to the grinding jig and then dressed to the desired form in four minutes with the diamond dresser tool. The optimum conditions for grinding with the invention wheel were found to be the maximum table speed of the machine, 16 /2 feed/min, using a downfeed of .0005 inch/pass for roughing and 0.00025 inch/pass for finishing. The results were an excellent finish, completely burr-free. The finish on the workpieces was in all ways comparable to that achieved with the customary metal bonded wheel, but the time required was much shorter and, therefore, the operation can be performed more economically. In six hours of testing, there was no measurable wear on the wheel.

In the electrolytic grinding of jet engine blades, another aluminum oxide wheel has given markedly better finish than the previously used metal bonded diamond wheel.

It can be seen from the above tests that the Wheel, in accordance with the present invention may, when used with diamond abrasive in the rim, be used either for conventional grinding or electrolytic grinding. The diamond bearing wheel has shown excellent performance in conventional grinding tests; when used in electrolytic grinding, it has been found possible to use substantially higher current densities without arcing and sparking, and the rate of stock removal is, thereby, greatly increased. When aluminum oxide is used as the abrasive, the wheel is limited as a practical matter in the fact that, whereas it may be used with some types of conventional grinding, it finds its best use in electrolytic grinding. The aluminum oxide wheel may, furthermore, be manufactured in the conventional one-piece manner. That is to say, instead of making a non-abrasive core and bonding to it an abrasive containing rim, the entire wheel may be made of one abrasive containing formulation. This, obviously, gives much longer wheel life, particularly in the case where face may easily be dressed to the desired form and its useful life greatly extended.

Wheels made according to this invention may weigh over three times more than an ordinary organic bonded wheel of the same size.

ing or chattering and bearing wear in machine. However, surprisingly enough, the extra weight apparently has a damping effect on machine vibration and an unusually fine smooth cutting action is thereby achieved.

While it will be apparent that the illustrated embodiments of the invention herein disclosed are well calculated adequately to fulfill the objects and advantages primarily stated, it is to be understood that the invention is suceptible to variation, modification, and change within the spirit and scope of the sub-joined claims.

The invention having been thus described, what is claimed as new and desired to secure by Letters Patent is:

1. An abrasive article, comprising:

(a) a core consisting of a powder of a substance having a thermal conductivity of 0.500 cal./cm. /cm./ sec./ C. at 68 F. or greater in an amount in the range from 80% to 95 by weight and the remainder resin, and

(b) a rim attached to the core consisting of to 85% by weight of the said powder, 8% to 16% by Weight of resin, and the remainder a material selected from the group consisting of diamond and aluminum oxide.

2. An abrasive article as recited in claim 1, wherein the substance is one selected from the group consisting of copper, aluminum, and silver.

3. .An abrasive article as recited in claim 1, wherein the powder is present in the core in the amount of 92% by weight.

4. An abrasive article for use in conventional and electrolytic grinding, comprising:

(a) a core consisting of to 95 by weight of a powder of a substance selected from the group consisting of copper and silver, 5% to 20% of resin, and

(b) a rim attached to the core consisting of 70% to of the said powder, 8% to 16% of resin, 1% to 20% of a material selected from the group consisting of diamond and aluminum oxide.

5. An abrasive article having a uniform composition, comprising a powder of a metal having a thermal conductivity of 0.500 cal./cm. /-cm./sec./ C. or greater in the range from 70% to 85 by weight, 8% to 16% by weight of organic bonding material, and the remainder abrasive.

6. An abrasive article having a uniform composition, comprising a powder of copper in an amount in the range from 70% to 85 by weight, phenol formaldehyde resin in an amount in the range from 8% to 16% by weight, and the remainder abrasive.

7. An abrasive article, comprising (a) a core having 80% to by weight of a powder of a substance selected from the group consisting of copper and silver and 5% to 20% of resin, and

(b) a rim attached to the core having 70% to 85% of a powder selected from the group consisting of copper and silver, 8% to 16% of resin, and 1% to 20% of a material selected from the group consisting of diamond and aluminum oxide.

References Cited by the Examiner UNITED STATES PATENTS 2,137,329 11/1938 Boyer 5l-209 X 2,150,886 3/1939 Van Der Pyl 51-206 3,074,211 l/ 1963 Sacco 51-206 ROBERT C. RIORDON, Primary Examiner.

various form grinding operations are desired. The wheel 75 L. S. SELMAN, Assistant Examiner.

Initially, it was feared that this 5 unusual weight would cause grinding problems in bounc- 1 

1. AN ABRASIVE ARTICLE, COMPRISING: (A) A CORE CONSISTING OF A POWDER OF A SUBSTANCE HAVING A THERMAL CONDUCTIVITY OF 0.500 CAL./CM.2/CM./ SEC/*C. AT 68*F. OR GREATER IN AN AMOUNT IN THE RANGE FROM 80% TO 95% BY WEIGHT AND THE REMAINDER RESIN, AND (B) A RIM ATTACHED TO THE CORE CONSISTING OF 70% TO 85% BY WEIGHT OF THE SAID POWDER, 8% TO 16% BY WEIGHT OF RESINN, AND THE REMAINDER A MATERIAL SELECTED FROM THE GROUP CONSISTING OF DIAMOND AND ALUMINUM OXIDE. 